Aging and many gait pathologies are characterized by reduced propulsive forces and ankle moment and power generation during trailing leg push-off in walking. Despite those changes, we posit that many individuals retain an underutilized reserve for enhancing push-off intensity during walking that may be missed using conventional dynamometry. By using a maximum ramped impeding force protocol and maximum speed walking, we gained mechanistic insight into the factors that govern push-off intensity and the available capacity thereof during walking in young subjects. We discovered in part that young subjects walking at their preferred speed retain a reserve capacity for exerting larger propulsive forces of 49%, peak ankle power of 43%, and peak ankle moment of 22% during push-off—the latter overlooked by maximum isometric dynamometry. We also provide evidence that these reserve capacities are governed at least in part by the neuromechanical behavior of the plantarflexor muscles, at least with regard to ankle moment generation. We envision that a similar paradigm used to quantify propulsive reserves in older adults or people with gait pathology would empower the more discriminate and personalized prescription of gait interventions seeking to improve push-off intensity and thus walking performance.